An electron capture detector (ECD) is a sensor used to analyze gas (analyte). The ECD takes several forms. All the forms are characterized by a flow through cell containing pair of spaced apart insulated field electrodes in a chamber and a source of ionizing radiation. As the analyte flows through the chamber, an "extraction voltage" waveform is applied between the field electrodes. In the absence of gaseous compounds which can capture the electrons, a given cell current is produced. The resultant electron density within the chamber is affected by attachment of electrons to the analyte. Electron density is measured using an appropriate transducer, e.g. electrometer, to measure the current generated by the application of the extraction voltage waveform to the field electrode. Usually the ECD cell is connected into an oscillator circuit where the pulse frequency is proportional to the analyte concentration. At higher frequencies the insulation leakage loss of the ECD cell increase at a faster rate and the linearity of the device is diminished. At low frequency the cell insulation leakage established a minimum detection current level. Constant frequency devices which measure concentration as a function of current are also known.
To minimize the leakage currents between the ECD cell field electrodes, very high resistance insulators are usually employed to isolate the pulsed voltage waveform from the electrometer circuit, i.e. to insulate the anode from the cathode. Typical resistance values are of the order of one million megohms. Specific theoretical models such as the Maggs model or the Wells model ignore the possibility of leakage currents clue to low resistivity components or contamination of high resistivity components which, in practice, effectively decreases the observed resistance between the anode and cathode structures. ECD's have a large linear dynamic range but they typically do not perform as well as the theoretical models. In addition, the ECD linear range is very sensitive to contamination of the cell outer surface and it is temperature sensitive. The insulator supporting and separating the field electrodes exhibits a leakage current across or through the insulators and this leakage is consequently input into the measuring transducer (e.g. electrometer) resulting in a decrease of linearity of the response of the electron capture detector. This distortion results for pulsed waveforms in either constant frequency or constant current modes of operation.